1. Field of the Invention
This invention relates generally to a machine executed method and a system for managing a multi-volume data set request in an automated storage data library. More particularly, the method and system allow for an efficient selection of an optimum peripheral data storage device candidate from among plurality of peripheral data storage devices in response to a request for access to a plurality of data volumes.
2. Description of the Related Art
Modem computer systems require large quantities of storage capacity. To meet this requirement computer memory is available in many forms. A fast, but expensive, form of memory is main memory, typically in the form of micro chips. Other available forms of memory, known as peripheral data storage devices (PDSDs), include magnetic direct access storage devices (DASD), magnetic tape storage devices, and optical recording devices. Each of these other types of memory has a greater storage density and lower cost than main memory. However, these other memory types require mechanical movement to access stored data and therefore exhibit slower access times than purely electronic main memory.
Storing all data in a computer system in main memory is too costly; but, storing all data on one peripheral storage device reduces performance. Thus, a typical computer system includes both main memory and one or more types of PDSDs arranged in a data storage hierarchy. In such a hierarchy, main memory is often referred to as the primary data storage. The next level of a hierarchy is known as the secondary data storage, and so on. Generally, the highest level of the hierarchy has the lowest storage density capability, the highest performance, and highest cost. As one proceeds down through the hierarchy, storage density generally increases, performance generally decreases, and associated costs generally also decrease. By transferring data between different levels of the hierarchy, as required, the cost of memory is minimized and performance is maximized. Data is thus stored in main memory only so long as it expected to be required by the processor.
Therefore, in order to have the required information available on an "as needed" basis, much storage at the lowest level of the hierarchy is required. Many business applications, for example, require numerous disks or tapes to meet the lowest level storage needs. Automated storage libraries have been developed to manage the storage of and provide efficient access to such disks or tapes.
Automated storage libraries include a plurality of storage bins or slots for retaining data storage media, such as magnetic tapes, magnetic disks, or optical disks, and also have a robotic picker mechanism, and one or more PDSDs. Each data storage medium may be contained in a cassette or cartridge housing for easier handling by the picker. The picker operates on command to transfer the storage media between the storage bins and the PDSDs without manual assistance. A PDSD having a storage medium mounted therein is referred to as "unavailable". Conversely, a PDSD without a storage medium mounted therein is referred to as "available". Once a data storage medium is mounted in a PDSD, data may be written to or read out from that medium for as long as the system so requires. Data is stored in secondary data storage in the form of one or more files, each file being a logical data set.
It is not uncommon for a large data set to span several hundred cartridges or cassettes in the case where the storage medium is tape. When the storage medium is a computer disk it is not uncommon for a data set to span more than one disk or more than one disk side. Each side of a cartridge, cassette or disk is considered a "volume". Data stored on one side comprises a "data volume". When a data set covers more than one side of a cassette or disk it is said to be a "multi-volume data set".
When a multi-volume data set is requested, the actual volumes containing the entire data set are likely to be dispersed in a plurality of storage bins located throughout the library. Due to the disperse locations of each volume in a multi-volume data set, selecting the optimum peripheral storage device or drive to transfer the data set to PDSDs greatly increases data access time and therefore decreases computer performance and efficiency.
Several automated storage libraries are known. IBM Corporation introduced the 3850 mass storage subsystem for retrieval of magnetic tape modules in the 1970s. More recently, several automated storage libraries for magnetic tape cartridges and optical disks have been introduced. Examples of an automated storage and retrieval system, employing an automated tape cartridge library can be found in U.S. Pat. Nos. 4,864,511 and 4,864,438. Examples of optical disk libraries can be found in U.S. Pat. Nos. 4,974,156 and 4,945,428. Also systems for general automated libraries capable of handling either tapes or optical disks are described in U.S. Pat. Nos. 5,121,483 and 5,197,055. The robotic picker mechanisms of these libraries include one or more grippers. Each gripper is capable of handling one data storage medium at a time. IBM Corporation also provides the 9246 Optical Library Unit including a two-gripper picker.
The efficient operation of pickers and grippers greatly increases the overall efficiency of an automated storage library. The selection of storage media for transfer between storage bins and PDSDs, the time of the transfers, and the selection of the location to be transferred to are among the factors which determine data access time.
U.S. Pat. No. 4,864,438 describes an apparatus for managing movement of tape cartridges in an automated tape cartridge library system. In the apparatus, each tape cartridge stored in the library system is identified and its physical location is determined, whether in its home position or in transit. The apparatus includes a path selection apparatus to manage the movement of the tape cartridges between home (storage) positions and mounting positions in designated PDSDs. In this regard, a path between two end points is selected, and the apparatus is reserved to transport the designated tape cartridges between these two end points. Provision is made for maintaining the apparatus reservations until the movement of the tape cartridge is completed. In this fashion, if the movement of the tape cartridge is blocked somewhere along the path, the apparatus remains reserved to return the designated cartridge back to its original or home position.
Careful selection of a mounting location might lessen the reservation load on the management apparatus because it would reduce the likelihood of the path being blocked. Specifically, if the cartridges were mounted on the nearest available drive then cartridge picker travel distances would be decreased. If cartridge pickers do not travel any farther than a minimum required distance then the likelihood of the travel path of one picker being blocked by another picker is decreased.
U.S. Pat. No. 4,864,511 discloses a storage and retrieval subsystem employing an automated tape cartridge library having a split controller architecture that permits the library management controller to operate transparently to a host computer, thereby eliminating the host computer overhead associated with picker control. The patent does not disclose how to select an optimum mounting location for mounting a storage medium.
Although the selection of a mounting location for a data volume is clearly critical, none of the other above mentioned U.S. patents discloses a method, apparatus, or system to prioritize and subsequently select the optimum mounting location to transfer the data storage medium containing a requested data volume.
A trend in automated library systems is to increase the amount of peripheral storage devices, such as drives. If the amount of drives is increased then there is an increased likelihood of not selecting a drive which is optimum with respect to the distance between itself and each volume of a multi-volume data set.
Another trend is to add redundant hardware to automated data library systems as part of a fail safe strategy and to increase performance. Such redundant hardware may include multiple robotic data storage medium pickers. However, when multiple robotic data storage medium pickers are available, the management of these pickers becomes critical. Managing the motion of two pickers, both of which travel on a common rail system, is not a simple task. If both pickers have to travel the entire length (or most of the length) of the rail simultaneously to process volume mount and demount requests, managing their movements to prevent their interfering with each other is difficult. If the travel paths of the two pickers begin to interfere with each other, such that one picker must wait for the second to move out of its current travel path, then reduced levels of performance can be expected because one picker will be idle waiting for the second to move out of the way. One would like to minimize the overlap of the travel path of the two pickers during normal operations. Accordingly, a method and system for minimizing the overlap and travel of the two pickers in an automated storage library is needed.